This invention relates to surface treatment of semiconductors and relates to a method and a device for treating a surface of a semiconductor as well as a semiconductor. It finds applications in the field of surface cleaning, etching and nano-manufacturing.
Techniques enabling to prepare silicon wafers with perfectly passivated and extremely plane surface, made of hydrogenated silicon, whereby hydrogen forms a monoatomic layer on top of silicon, are known. Such a preparation can be made from a silicon wafer, cut according to the standard practice of the man skilled in the art and exposed to open air. We know that surrounding oxygen covers immediately the surfaces of the wafer in order to oxide silicon while forming SiO2 molecules over a typical thickness of 5-6 atomic layers corresponding to approx. 20 Angstrom. Thus, according to a method rather well known by the specialists and produced by BELL TELEPHONIE Co., the surface-oxidised silicon wafer is dipped into two successive baths, whereas the first one contains hydrofluoric acid and the second one hydrofluoric acid as well as ammonium ions. Treated surfaces are thus etched and covered with a single hydrogen layer. The resulting wafer is kept under vacuum, protected from external contaminations.
Such techniques enable therefore to obtain extremely clean surfaces, but with an external hydrogen-passivated layer. Still, it often proves useful to have one or several external insulating layers, notably made of SiO2. For instance, a silicon wafer covered with an SiO2 layer can be used for manufacturing an MOS-type transistor, by application of an additional metallic layer.
Another problem associated with surface treatment of semiconductors relates to the etching of these surfaces (lithography). Surface marking at industrial level is currently performed by a fraction of a micron, as regards the sizes of the marks as well as their positions. It has been suggested to send highly charged ions to a mica or graphite surface, in order to create permanent punctual defects using contact and highly energetic collisions. These defects build blisters of a few nanometers in size, distributed in such a way as to change the surface topography.
Major shortcomings of this etching method are that these methods only modify the surface topography.
The document E-A-0.567.985 discloses a method for manufacturing a thin film. This method comprises a first step consisting in selective irradiation of a substrate whose surface has been treated to contain hydrogen atoms so as to form an irradiated zone and a non-irradiated zone on this surface. The method comprises a second step consisting in selective formation of a thin film on the non-irradiated zone. As shown by this document, the disclosed method enables drawing up on a semiconductor substrate thin aluminium films whose thickness is liable to be as small as 100 nm.
The document patent Abstract of Japan, vol. 17, No. 649 (E 1468), JP-A-5.211.120 describes a device enabling to form a sample of thin film while adjusting the kinetic energy of an ionic beam originating from a liquid source of metallic ions and sent toward a target. The formation of the sample at the surface of a substrate is obtained by irradiating the substrate using the ionic beam.
The document Applied Physics Letters, vol. 56, No. 20 (May 14, 1990), pages 2001 to 2003, JA. DAGATA et al., mentions chemical modification of passivated silicon surfaces with hydrogen, using an STM-type tunnel effect microscope. This document mentions a 100 nm-line resolution.
This invention relates to a method for treating a surface of a silicon wafer, enabling to make this surface extremely clean and plane, and made by one or several SiO2 monolayers.
More generally, the invention relates to a method for treating a semiconductor surface, enabling to make this surface extremely clean and well delineated, and made by an isolating compound extending over one or several molecular layers.
The aim of the invention is also to provide a method for treating a semiconductor surface, enabling to etch the said surface by approx. one nanometer.
The invention relates to such methods that can be implemented easily, enable real-time control of the surface preparation and make later control very simple as well as extremely accurate.
Another aim of the invention is a device for treating a semiconductor surface, enabling to clean this surface to make it extremely clean and well delineated, and made by an insulating compound over one or several molecular layers.
The invention also relates to a device for treating a semiconductor surface enabling to etch this surface by approx. one nanometer.
The invention relates to such treating devices that can be performed and implemented quite easily, which enable real-time control of the surface preparation and later control of the surface condition, simply as well as extremely accurately.
The invention also relates to a semiconductor with considerable information storage capacities, preferably by a factor at least equal to 10,000 times those of existing semiconductors.
In this view, the invention relates to a method for treating a semiconductor surface, whereby this surface is made by first molecules of the semiconductor with external bonds and the said bonds are saturated with hydrogen atoms.
The method comprises the steps of:
generating positive ions each having at least three positive charges, with low energy, with respect to the MeV,
sending under vacuum a beam made of these ions toward at least one zone of the surface, whereas these zones cover a determined portion of the surface;
applying to the beam a deceleration voltage close to the surface, in order to give the ions of the beam a controlled average speed, whereas the ions extract electrons of the first molecules of these zones without contacting the surface, so that these first molecules lose their hydrogen atoms and that the corresponding external bonds become pendant, and
sending toward these zones a product saturating the pendant external bonds in order to form second molecules of an insulating compound.
By xe2x80x98surfacexe2x80x99, we mean a superficial part of the semiconductor, generally cut approximately along a crystallographic plane.
The surface is advantageously plane, in order to serve notably as a substrate for growing layers or for etching. In embodiment variations, the surface is curved.
By xe2x80x98highly chargedxe2x80x99 positive ions, we mean ions with at least three positive charges and preferably at least fifteen positive charges. Their energy is said to be xe2x80x98lowxe2x80x99 with respect to that of ions obtained using a particle accelerator, whereas this energy is approx. one MeV or one GeV. The low energy of the ions is thus smaller than a few tens keV.
The xe2x80x98vacuumxe2x80x99 under which the beam of ions is sent may correspond to relatively high pressure, for instance in the order to 10xe2x88x929 Pa. It can also be ultrahigh vacuum.
The deceleration voltage is applied in order to give very low energy to the ions, close to zero and generally smaller than a few tens eV.
An important aspect of the treatment method according to the invention is that the ions do not contact the surface. On the contrary, they attract surface electrons, then flow away in the opposite direction.
Extracting electrons from a semiconductor by using highly charged ions with low energy is explained in the article by Jean-Pierre BRIAND presented at the Fourteenth International Conference of Accelerator Applications in Research and Industry, DENTON-Texas, 6-9th November 1996. Diagrammatically, a highly charged ion with low energy starts to interact with the semiconducting medium at quite a long distance from the surface, that can reach a few tens Angstroms. The ion attracts and then captures conduction or valence electrons that go through Rydberg conditions. The ion thus becomes a hollow atom, i.e. an atom with internal layers, which are at least partially empty, and with external layers made by energised electrons. The number of electrons captured by the ion is considerably higher than its charge, since a portion of these electrons are then expelled from the ion by Auger effect. The number of electrons removed from the semiconductor by a single ion is generally equal to approx. three times its charge.
Close to the surface, the ion generates an electric image, which exerts an attraction force on the ion and thus tends to accelerate its movement toward the surface. However, the extraction of electrons by the ion creates, on semiconductors or on insulators, positive holes at the surface which compensate for this electric image. The hollow atom formed from the ion can be back scattered without making any contact above the surface, by xe2x80x98trampoline effectxe2x80x99. The existence of contact and of ingress or not inside the semiconductor material depends on the initial cinematic conditions of the ion: above a critical speed, the ion directed toward the surface reaches and penetrates the semiconductor material in spite of the formation of positive holes. Conversely, the trampoline effect occurs below this critical speed. The critical speed has a value that depends on the extraction potential of the semiconductor material and on the initial charge of the positive ion.
Controlling the average speed of the ions using the deceleration voltage enables to trigger the trampoline effect and to give the ions controlled charge and energy.
Moreover, each ion extracts electrons from an elementary interaction zone whose surface is determined by the position and the speed of the ion. The determined zone toward which the ion beam is sent thus consists of the association of all the elementary interaction zones of the ions in the beam.
The electrons extracted from the surface of the semiconductor are essentially electrons associated with the external bonds of the first molecules. In their absence, the hydrogen atoms saturating the external bonds are reduced to protons that are not bound to the surface any longer. The external bonds then become pendant.
Etching the surface of the semiconductor can be perfectly controlled spatially, by controlled position of the ion beam enabling accurate selection of the etching zones. Preferably, etching takes place in successive stages, one zone after the other, while varying the position or the orientation of the beam or of the target. According to an embodiment, several beams are directed toward the surface simultaneously.
The treatment method according to the invention differs from the existing etching techniques by the absence of any contact between the ions of the beam and the surface of the material. By contrast to the method consisting in forming blisters by ion shocks, the topography of the surface is not modified but its conductivity is. Indeed, whereas the first molecules are semiconducting, the second molecules are insulating.
In the field of etching, the method according to the invention thus enables to generate insulating peaks in the order of one nanometer. Information storage capacities can thus be increased by 1002 even 10002 with respect to existing techniques. Moreover, local control of the surface conductivity after etching is easy and quick, using a tunnel-effect microscope.
In the field of surface cleaning, the treatment method according to the invention enables to obtain an extremely clean and well-defined surface, made by the second molecules of the insulating compound.
According to a first embodiment of the treatment method of the invention, the zones cover the whole surface.
The treatment method then consists in modifying the semiconducting surface into an insulating surface and is used preferably within the scope of surface cleaning.
According to a second preferred embodiment of the treatment method of the invention, the zones delineate an insulating network etched at the surface of the semiconductor.
This network comprises the second formed molecules, adjacent the first molecules left in place. This second embodiment enables to etch the semiconductor.
Preferably (in both embodiments), after having formed the insulating compound in the zones, electric conductivity is controlled locally using a tunnel effect microscope.
This control offers necessary accuracy in view of the finesse of the treatment. The surface can be read electrically using the tip of a tunnel effect microscope scanning this surface. By xe2x80x98local controlxe2x80x99, we mean permanently localised control, that can be applied by spatial scanning to any portion of the surface.
Advantageously, for prior preparation of the semiconductor surface from the oxidised condition of the surface, the method comprises the steps of:
dipping the semiconductor into a first hydrofluoric acid bath, then
dipping the semiconductor into a second bath, composed of hydrofluoric acid and ammonium ions.
This prior step, which corresponds to the technique used by BELL TELEPHONIE Co., enables to switch from the oxidised condition of the surface implementing five to six oxygen atomic layers at the perfectly controlled surface with a single atomic layer of hydrogen atoms.
Advantageously, the product comprises oxygen saturating the pendant external bonds.
The second formed molecules thus exhibit electrical insulation properties and the semiconductor can be used as a substrate for controlled growth of additional layers from a first layer of the second molecules, for instance to manufacture an MOS transistor or a capacitor network by metal deposit.
It is then interesting to send the beam of ions under partial vacuum, so that oxygen saturating the pendant external bonds is sent jointly.
Thus, it is not necessary to provide a specific and independent step for sending the product since the latter is spontaneously available close to the surface in the form of oxygen. Partial vacuum can reach 10xe2x88x929 Pa.
According to another embodiment for sending the product, the beam of ions is sent under very high vacuum so that oxygen does not saturate the pendant external bonds and then, after etching of the surface, the product carrying the atoms saturating the pendant external bonds is sent. This product can be nitrogen for example. The process operations according to the invention are made under ultrahigh vacuum, for example in the order of 10xe2x88x929 Pa.
In a preferred embodiment of the method of the invention, the semiconductor is based on silicon.
The first molecules are then essentially SiH. The second molecules, for their own parts, are SiO2 in case when the product brings atoms of oxygen and silicon nitride (Si3N4) in case when it brings atoms of nitrogen.
Preferably, electrons are extracted by the ions while measuring the X-rays emitted when the extracted electrons switch from one electronic layer to another electronic layer of the ions.
Indeed, the electrons captured by hollow atoms and non-expelled flow down toward deeper layers while causing the emission of X-rays. These phenomena are described in the article by Jean-Pierre BRIAND previously mentioned, as well as in an article by Jean-Pierre BRIAND et Coll., published in Images de la Physique, 1992, pages 58-62. In a simplified way, emission and detection of X rays take place as follows. Only the last X transitions emitted in the deep layers are observed, whereas the transitions in the higher layers exhibit lower energy and are immersed in the continuous background. The transitions measured are therefore preferably those toward the L and K layers. The gaps in the L and K layers are filled up gradually. In the treatment method according to the invention, the hollow atoms are formed in Rydberg conditions so that a very large number of steps are necessary to reach the deep layers. The L layer then exhibits a low filling speed with respect to that of the K layer so that a Kxcex1-type X-ray is emitted as soon as an electron reaches the L layer.
Gradual filling of the L layer or more generally of internal layers is used advantageously for temporal measurement of events, by detection of emitted X-rays. Filling up the internal layers indeed generates temporal marks spaced by a few ten femto-seconds.
Advantageously, for controlling X-rays by measurement, deceleration voltage is caused to vary and the X-rays emitted are measured for several values of the deceleration voltage.
The purpose of the invention is also a device for treating a semiconductor surface, whereas this surface is made by the first molecules of the semiconductor with external bonds and hydrogen atoms saturate these bonds.
According to the invention, the treatment device comprises:
a source of highly charged positive ions with low energy,
means for applying an emission voltage extracting a beam of ions of the ion source and directing the said toward the surface,
means for applying a deceleration voltage, arranged close to the surface in order to give the ions of the beam a controlled average speed, enabling the ions to extract electrons from several of the first molecules without contacting the surface and thus causing the first molecules to lose their hydrogen atoms and to make the corresponding external bonds pendant,
a source of a product saturating the pendant external bonds in order to form second molecules of an insulating compound, whereas this source sends the product toward the surface further to a passage of the beam, and
a device for depressurising the beam and the surface.
The treatment device according to the invention can be used for cleaning or for surface etching.
It enables to obtain an extremely clean and well-defined surface, covered with an insulating layer or an etching with an insulating pattern on a semiconductor background.
Preferably, the treatment device according to the invention comprises a tunnel effect microscope, performing local electrical conductivity control of the surface treated.
Preferably, the treatment device according to the invention comprises an instrument for measuring emitted X-rays, when electrons extracted from an electronic layer switch to another layer of these ions.
The invention also relates to a semiconductor with a surface in which an insulating network is etched. According to the invention, the insulating network is etched approx. one nanometer deep.
The method according to the invention provides a means to obtain such a nanocomponent.
Preferably, the surface after etching is covered with a conducting coating.